This invention relates to the use, in the storage and retrieval of information, of a multilayer film comprising different types of layers which are distinguishable on the basis of their different optical properties. The multilayer film has application as an anti-counterfeit device.
Counterfeiting is a problem that is growing world wide and is costing the global economy over $250 billion a year. Many products are affected including credit cards, documents, apparel, videos, auto parts and aircraft parts.
One commonly used device for storing information is the barcode. It is known that barcodes may be expressed as mathematical code when optically scanned using appropriate equipment and software. Hand-held scanners are typically used. This mathematical code can then be correlated with a code stored in a database for retrieval of the desired information or for verification purposes. The database may be a local database situated at the point of scanning or it may be located remotely and accessed according to known means of telecommunication. One disadvantage of the barcode is that because the structure of the barcode is readily discernible, it is easily reproducible. For this reason, a conventional optically-scanned barcode is not particularly suitable for use as an anti-counterfeit device.
Many other existing anti-counterfeiting techniques, such as holograms, rely on methods that are visible to the human eye. It would be desirable to provide a technique involving a code which is invisible to or indiscernible by the naked eye and therefore much more difficult to copy. One fairly recent development in high security applications involves sub-surface laser marking within a label. Although this method provides a high degree of security, it involves a relatively complicated and complex process of production and data retrieval.
Polyester film is already used as a substrate for cards and label applications. A security feature present as an integral feature of the film would enhance the utility of polyester film in such applications. A polyester film within which is embedded a code that is invisible to the naked eye would have particular utility in such applications.
The preparation of multilayer films is known. U.S. Pat. No. 3,647,612 discloses a process for the preparation of a multilayer film which involves providing two or more streams of thermoplastic material, arranging the two or more streams into a single stream having a plurality of generally parallel layers, mechanically manipulating the stream by dividing and recombining to provide an increased number of layers and then forming the stream into a thin sheet or film. The layers are composed of resinous material which is transparent to visible light, the multilayer film structure having an iridescent appearance. EP-0426636 also discloses multilayer coextruded light-reflecting films comprising a plurality of generally parallel layers of transparent thermoplastic resinous material.
EP-0592284 discloses a tear-resistant multilayer film comprising alternating layers of stiff and ductile polymeric materials which may be useful as a laminate for shatter-proofing a glazing member.
U.S. Pat. No. 5,759,467 discloses a multilayer polyester film which comprises a plurality of alternating layers of terephthalic acid polyester and naphthalene dicarboxylic acid polyester. The film has increased tensile strength and is taught as being of use in, inter alia, magnetic media substrates.
EP-0492894 discloses a method and apparatus for the production of a multilayer film by generating interfacial surfaces in a fluid mass. The method comprises the steps of dividing a first composite stream into at least two branch streams, repositioning the branch streams, expanding symmetrically along one axis, contracting symmetrically along another axis and recombining the branch streams into a second composite stream which comprises a greater number of discrete layers of polymeric material than the first composite stream, wherein the expansion and contraction steps are conducted either on the individual branch streams or on the second composite stream.
WO-98/06587 discloses a polyester film having an opaque, preferably black, core layer having an optical density greater than 2.0 and on both surfaces thereof a white outer layer for use as a photographic sheet or other imaging applications. For similar uses, WO-98/07068 discloses a polyester film having an opaque, preferably black, first layer having an optical density greater than 2.0 and a white second layer.
It is an object of the invention to provide a convenient and inexpensive technique for combating fraud which offers a high level of security and which has a high resistance to counterfeiting.